In magneto-optic recording systems, digital information is stored in a thin magnetic storage medium by locally magnetized regions or domains. The regions are magnetized to represent either ones or zeros. The information is written into the magnetic storage medium by raising the temperature of localized small regions of the magnetic medium to the Curie point temperature of the medium at the localized regions. This lowers the coercivity to a point which enables orientation of the magnetic domain by an external magnetic field. The size of the regions or domains determine the density of the digital information. The size of the localized region is limited by diffraction and is marginally improved by use of shorter wavelengths of light and higher numerical aperture lens. Stored information is read by Kerr or Faraday rotation of a polarized light beam incident on the magnetic medium by the magnetic fields at the magnetized regions or domains. The shift in polarization is in the order of 1 degree. This shift is employed to detect ones and zeros. Systems for reading out these small rotational changes are well established in the optical storage industry. Optical recording and the design of conventional read/write heads is described in the book entitled "Optical Recording" authored by Alan B. Marchant, Addison-Wesley Publishing, 1990.
Betzig and others have overcome the diffraction limitation definition by employing near-field optics. They have demonstrated orders of twenty nm or better for the magnetized regions or domains. (E. Betzig, J. K. Trautman, R. Wolfe, P. L. Finn, M. H. Kryder and C. H. Chang, "Near-Field Magneto-Optics and Hi-Density Data Storage", Appl. Phys. Lett. 61, 142-144, (1992)). The basic idea of near-field optics is to pass an optical beam into a metal covered optical fiber which is tapered down to a small size with a pinhole at its end. If this pinhole is placed close to the object being illuminated or imaged, in this instance the magnetic media, the definition is controlled by the size of the pinhole, rather by diffraction limits.
The problem with the use of a tapered fiber is that it does not propagate waves in the region where the diameter of the fiber is less than approximately 0.3 wavelengths of the light. Propagation through wave guide in this region is cut off and the loss of energy is extremely high, of the order of 30 dBs. Thus the amount of light energy which is applied to the medium is limited and heating of the medium to the Curie temperature requires a finite time. This makes it impractical for use with high speed storage systems. For example, with light penetration into the magnetic media of about 15 nm and an illumination wavelength of 546 nm, using quartz with fiber reflective index of 1.5, the minimum effective size of the beam in the propagating region of the quartz fiber is 140 nm. If the fiber is tapered to a size much smaller than this amount, the attenuation in the cutoff region is very high. Because of this high attenuation, the technique is unsuitable for optical storage at high data rates.